The proposed research can be grouped as follows: 1. Determination of the diffusing capacity of the placenta for CO (DP sub CO) for pregnant women, by measuring the exchange of CO between the mother's circulation and that of the fetus. 2.) Estimation of the effect of a given blood (HbCO) on tissue oxygenation of the fetus in animals by following signs of fetal distress and indices of anoxia, such as venous PO2 . 3.) Measure the effect of changes in the level of gonadal steroids on CO production in women, and investigate the mechanisms of its increase during gestation. 4.) Attempt to measure V sub CO in the fetus by following changes in blood (HbCO) after the introduction of damaged red cells. 5.) Study the ion exchanges across the placenta by applying the voltage- clamp technique. 6.) Measure the effect of changes in blood pH, P sub CO2 and pertinent drugs on placental blood flow using animals with chronically implanted electromagnetic flowmeters.